1. Field of the Invention
The present invention relates to a printing apparatus which ejects ink to carry out printing and to a method for controlling printing apparatus More specifically, the invention relates to a printing apparatus which carries out a recovery operation by circulating ink between a print head and an ink tank and to a method for controlling the printing apparatus.
2. Description of the Related Art
An ink jet printing apparatus ejects fine ink droplets from a nozzle of a print head on a print medium such as a sheet of paper so as to print characters and images on the print medium. The conventionally known ink jet printing apparatus, in order to prevent the defective ejection, removes ink in the nozzle whose viscosity increases and then supplies new ink into the nozzle, or carries out the recovery operation of an ink jet print head by eliminating microbubbles mixed in the ink. As a specific method for recovery operation, there is a preliminary ejection recovery operation method in which ink droplets are ejected from a nozzle of the print head on a waste ink receiver such as a cap, and an ink pressure recovery method in which pressure is applied to ink in the nozzle to forcedly discharge the ink on the waste ink receiver such as a cap. There is also a suction recovery method in which ink is sucked out from the nozzle by, for example, a suction cap.
Japanese Patent Laid-Open No. H08-244250 (1996) discloses a printing apparatus having two pumps in a circulation passage, and provided with two ink passages on both inlet and outlet sides of the print head for connecting a print head to an ink tank, wherein at the time of terminating the circulation operation, the pump on the outlet side is stopped in advance, followed by the stop of the pump on the inlet side. Therefore, the pressure is applied to the inside of the print head to eliminate bubbles from the nozzle in the print head.
A line-type print head has been conventionally known in which a plurality of nozzle chips is aligned in a regular manner such as a staggered arrangement. If the pressure recovery method disclosed in Japanese Patent Laid-Open No. H08-244250 (1996) is applied to such an elongated print head having the structure as mentioned above, the problem arises as follows. A pressure loss within the ink passage in the print head generates the difference in ink discharge timing between a nozzle of a nozzle chip near a pressure recovery source (such as a pump) and a nozzle of a nozzle chip far from the pressure recovery source when the pressure recovery operation is carried out. The nozzle of the nozzle chip near the pressure recovery source discharges ink earlier, and a delay occurs in discharge of ink from the nozzle of the nozzle chip far from the pressure recovery source.
Therefore, in the case where the print head discharges the necessary amount of ink for removing the ink whose viscosity increases and eliminating microbubbles, the nozzle near the pressure recovery source discharges larger amount of ink than that primarily required if the nozzle far from the recovery source discharge necessary amount of ink. As described above, the larger amount of ink discharged than that primarily required for removing the ink whose viscosity increases and eliminating microbubbles results in the waste of ink consumption. Furthermore, since ink spills out of the nozzle at the time of pressure recovery operation, the following wiping process for wiping the ink from the nozzle surface is required. At that time, if more ink spills out than necessary, a single wiping operation cannot completely wipe out the ink on the nozzle surface, and thus the ink may remain on the nozzle surface.